The discussion up to this point has focused on the embryonic development of the diaphragm. The diaphragm of the embryo and the newborn, however, is remarkably different from that of the adult or even the toddler, as are most organs and muscles of the body. Thus, many studies have been done on the postnatal development of the diaphragm in humans, sheep, and rats. Below are briefly outlined the main findings of these studies, which show how the diaphragm changes postantally in form and functioning capacity.
The infant diaphragm looks remarkably different from that of an adult. The infant diaphragm is only slightly curved and does not yet reach to the entire caudal part of the rib cage; thus it is both shaped and positioned to act as a bellows. The adult diaphragm, on the other hand, has a definite dome shape. This fibrous dome is perpendicular to the anterior-posterior axis, and the muscle fibers are positioned parallel to the anteroir-posterior axis. The adult diaphragm is attached to the rib cage along its entire circumference, except where the two crurae attach to the lumbar vertebrae. These changes arise from the ribs' downward movement, which brings the costal parts of the diaphragm with them. Eventually the diaphragm reaches a level directly anterior to the liver. The abdominal muscles strengthen and keep the liver in its relatively anterior position, which contributes to the increasing dome shape of the diaphragm. (Devlieger et al., 1991)